Human Immunodeficiency virus type 2 (HIV-2) represents one of two major classes of human lentiviruses causing Acquired Immunodeficiency Syndrome (AIDS) in man. Although genetically and biologically related to HIV-1, HIV-2 possesses a number of properties that distinguish it from HIV-1 and make it a particularly attractive target for molecular studies of HIV biology and pathogenesis. These include a close genetic and epidemiologic relationship to a non-pathogenic simian lentivirus infecting feral sooty mangabeys in West Africa, virulence properties ranging from relative attenuation in certain human populations to high level pathogenicity in others, and the availability of an animal model system (macaques) for the evaluation of molecular determinants of viral pathogenicity. In this renewal application, we propose to systematically evaluate the molecular basis for attenuated virulence which we have identified in strains of HIV-2 infecting healthy individuals from rural or remote rural parts of West Africa. Functional analyses of genes and regulatory regions amplified directly from infected primary (uncultured) patient material will allow us to determine whether such viruses differ from prototypic strains in their fundamental viral properties and whether such viruses represent HIV-2 substrains with significantly reduced pathogenic potential. Specific aims of the project are: 1. To employ nested PCR techniques to amplify and characterize individual HIV-2 genes and regulatory regions directly from uncultured PBMC DNA of healthy individuals from whom virus isolation has been repeatedly unsuccessful. 2. To use chimeric proviral constructs to functionally analyze the biologic activity of such genes and regulatory regions upon infection of natural target cells in vitro. 3. To utilize recombinant vaccinia virus expression systems to evaluate independently the structure/function relationships of envelope glycoproteins from "culture-resistant" viruses, to determine whether they are deficient in processing, transport or cell surface expression, to test the extent of their cytopathic and fusogenic potential, and to determine their CD4 binding affinity. 4. To functionally analyze the reverse transcriptase of HIV-2 strains characterized by G to A hypermutation, to determine the genetic mechanisms responsible for this hypermutation, and evaluate its biological significance and role in the natural history of HIV-2 infection. 5. To obtain and characterize additional field strains of HIV-2 to confirm the validity of findings made in aims 1-4 and to obtain, by a similar approach, field strains of HIV-1 from remote African jungle areas for similar evaluation. From these studies, we expect to identify clinically important virologic determinants of HIV pathogenesis and insights into factors associated with the recent transmission of these viruses from simian species to man.